1. Field of the Invention
The present invention relates to an apparatus and a method for automatically generating thumbnail images in motion picture video contents.
2. Description of the Related Art
Thumbnail images are used to provide a variety of functions, such as efficient contents summarization, storyboards, key frame generation and program guide in a personal video recorder (PVR) of a digital TV or a video browser of a video library.
The thumbnail images which display a specific video frame in a smaller size than an original size are used in various fields to summarize video contents recorded in the PVR into a few small pictures or to generate key frames for video indexing or representative images for program guidance.
Conventional methods for generating thumbnail images include (1) a method for decoding a video frame encoded by MPEG and reducing the restored frame, (2) and a method using a DC value of an I picture.
In the former, the whole pixels of the corresponding frame must be decoded, and thus a variable length decoder is required, which increases hardware load.
In the latter, the variable length decoder is not necessary because thumbnail images are generated by using the DC value of the I picture. The latter method has been more widely used due to its simple construction.
The DC value will now be explained.
Coefficients of a frequency region transformed by DCT in an image encoding process are classified into DC or AC coefficients according to the related frequency. Here, the DC value, which is meant as a coefficient when the frequency is ‘0’ implies an average.
That is, the DC values of each block in 8×8 block DCT are equivalent to averages of brightness or chrominance prior to the DCT.
Since the thumbnail images allow users to summarize and display the whole or specific section video contents in a smaller size than an original size, namely representative images of low resolution, the video frame composing the thumbnail images must be free from noise, blank and overlap due to scene transition or shot transition, and must be able to clearly transmit information of the whole or specific section video contents to the users. Therefore, avoided are unclear images influenced by blur due to camera movement such as rapid zoom and object movement.
In the case that the thumbnail images are generated in all I pictures of video, a large volume of thumbnail images are generated to increase load of a storing medium. In addition, the users have difficulties in searching wanted contents because of a lot of unnecessary thumbnail images. Considering that a number of the thumbnail images that can be displayed on one screen are restricted, it is difficult to efficiently summarize and display the whole or specific section video contents.
In order to solve the foregoing problems, there is provided a method for generating thumbnail images at a predetermined interval in a sample rate, which has been previously decided or optionally inputted by users. However, the aforementioned method, which simply restricts a number of the thumbnail images cannot overcome noise, blank and overlap of the thumbnail images.
It is thus impossible to prevent generation of unnecessary thumbnail images which are not suitable for information transmission.
There has also been taught a method for designating a section for generating thumbnail images in video. However, users must manually search the corresponding section.
Moreover, the conventional method for generating the thumbnail images does not consider a concept of scenes.
That is, when the thumbnail images are generated in every I picture, generated at a predetermined time interval in a designated sample rate, or generated in a section designated by users, a few similar thumbnail images are extracted from the same scene. It means that the users are provided with unnecessary thumbnail images having low information transmission ability. As a result, the representative images corresponding to each scene are not efficiently extracted.